A Water‐Soluble Highly Oxidizing Cobalt Molecular Catalyst Designed for Bioinspired Water Oxidation

Abstract

Herein, we present a stable water-soluble cobalt complex supported by a dianionic 2,2'-([2,2'bipyridine]-6,6'-diyl)bis(propan-2-ol) ligand scaffold, which is a rare example of a high-oxidation species, as demonstrated by structural, spectroscopic and theoretical data. Electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility measurements revealed that the Co IV center of the mononuclear complex in the solid state resides in the high spin state (sextet, S = 5/2). The complex can effectively catalyze water oxidation via a single-site water nucleophilic attack pathway with an overpotential of only 360 mV in a phosphate buffer with a pH of 6. The key intermediate toward water oxidation was speculated based on theoretical calculations and was identified by in situ spectroelectrochemical experiments. The results are important regarding the accessibility of high-oxidation state metal species in synthetic models for achieving robust and reactive oxidation catalysis.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

“…In a similar approach, a cobalt catalyst 2 (Figure ), based on a bipyalk ligand, was investigated with low-temperature EPR and UV–vis spectroelectrochemistry to gain information about the electronic structure and reaction mechanism. Detailed EPR spectroscopic analysis showed the Co metal center to exhibit a +4 valence state with a high-spin sextet ground state of 5/2 . The flexibility and electronic stabilization offered by the ligand scaffold were shown to sustain the Co­(IV) state over the water oxidation cycle, which proceeds via an PCET mechanism as supported by DFT calculations.…”

“…Detailed EPR spectroscopic analysis showed the Co metal center to exhibit a +4 valence state with a high-spin sextet ground state of 5/2. 769 The flexibility and electronic stabilization offered by the ligand scaffold were shown to sustain the Co(IV) state over the water oxidation cycle, which proceeds via an PCET mechanism as supported by DFT calculations. Horner et al deduced the detailed O−O bond formation pathway of a non-heme pentadentate iron complex 3 (Figure 61) via UV−vis−NIR spectroelectrochemistry and isotope labeling experiments.…”

Coordination-Adaptive Molecular Platform for Catalytic Water Oxidation

“…The successful use of bipyridine-based anionic ligands for first-row transition metals has been demonstrated in both experimental and theoretical studies, yielding three distinct types of catalysts. In 2022, Chen and Yu designed a Co-bdalk catalyst with the axial positions occupied by two acetate ligands (Figure a, left), which underwent ligand exchange with aqua ligands to initiate the catalytic cycle, differed from Ru-analog with the formation of 6- or 7-coordinate in the equatorial position, exhibited a TOF of 1.5 s –1 and an overpotential of 360 mV at pH 6. The incorporation of copper into bipyridine amidate-based ligands led to a series of adaptive catalysts displaying distorted square planar geometries (Figure a, middle) .…”

Identification, Characterization, and Electronic Structures of Interconvertible Cobalt–Oxygen TAML Intermediates

“…Recently, several reports have detailed the effectiveness of molecular cobalt complexes as catalysts for water-splitting reactions. − Although the homogeneity of these reactions has been convincingly established in many cases, − the harshness of the reaction conditions and the transient nature of some intermediates made it challenging to establish the structural properties of the species involved along the catalytic pathway, which resulted in an ambiguous reaction mechanism. − The interconversion between cobalt-oxo and cobalt-hydroxo species − is particularly relevant for their proposed role in artificial water oxidation. However, there is a lack of direct evidence for either of these species under catalytic turnover reaction conditions.…”